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Содержание HF-250

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Страница 2: ...s Introduction 2 Getting started 3 Controls and Connections 12 Operatingthe HF 250 17 Care of your Receiver 30 General notes 32 Optional Units 33 Circuit description 34 Receiver specification 36 Circuit Diagrams 44 C 1995 Lowe Production Ltd Bentley Bridge ...

Страница 3: ...d by a compromise in actual performance and the results obtained from some of these simple receivers have been quite disappointing The design and development of the HF 250 was based on straightforward objectives To obtain sufficiënt RF performance for the receiver to operate without problem in crowded bands with many strong signals To combine complete control of a necessarily complex piece of equi...

Страница 4: ...ternal loudspeaker a small bookshelf type Hi Fi unit is satisfactory We can provide a suitable unit as an optional accessory with the correct connecting lead for the HF 250 Any external loudspeaker should have an impedance of 4 to 8Q Record Output Many keen listeners like to tape record any interesting stations they hear and a low level audio output has been provided also a pair of relay contacts ...

Страница 5: ...ter provides a good compromise for most medium wave conditions and the 4 kHz filter for short wave The 2 2 kHz filter can be used under severe conditions but it is really only suitable for speech reproduction When AM mode is selected on the HF 250 the 7 kHz filter is initially switched in If you want to change to a different filter you can use the filter select function You may find that reception...

Страница 6: ... data systems are also very good CW signals are received in the same way as SSB signals with the carrier inserted in the receiver producing a beat note with the incoming signal In the CW mode the HF 250 provides an 800 Hz offset between the display and the intemal carrier so that a note is heard at 800 Hz when the receiver is tuned exactly to the signal The HF 250 initially selects the 2 2 kHz fil...

Страница 7: ...cy selective fading If a selective fade reduces the carrier level of an AM signal but leaves the sideband levels unaltered a receiver with a conventional AM detector will not be able to correctly reproduce the signal and the output will be distorted There are two techniques that can be used to improve the situation ECSS and Synchronous or Phase Locked AM detection ECSS Exalted Carrier Selectable S...

Страница 8: ...band signals Upper sideband AM For reception of AM broadcast signals AMS For AM reception using the synchronous detector DSB USB LSB DU 250 option AMSU Synchronous AM reception using upper sideband DU250 option AMSL Synchronous AM reception using lower sideband DU250 option FM For reception of narrow band FM signals DU 250 option Press this button and the mode indicators will flash Now use the MHz...

Страница 9: ...0kHz In CW mode two 2 2kHz and 200Hz In FM squelch facility on and off 11 16 Megahertz fast tuning buttons Tunes the receiver in one megahertz increments up or down the HF spectrum The tuning will continue automatically if either of the buttons is held pressed and will wrap around when at the end of its coverage 12 Memory mode select button Switches the frequency display into memory mode and then ...

Страница 10: ...e that is unaffected by the volume or tone control settings The level is suitable for feeding into the line input of most tape recorders and for driving RTTY decoders 24 External loudspeaker jack socket For connection of an external loudspeaker of 4 or 8 Q impedance Inserting a plug into this socket will disconnect the internal loudspeaker 25 12V DC power input socket For connection of a suitable ...

Страница 11: ...s The rate at which the receiver tunes when the tuning knob is rotated depends on the mode selected and on the speed of rotation of the knob All modes offer the facility of speed up tuning when the tuning knob is rotated rapidly the tuning rate increases This allows a slow tuning rate for precise signal resolution coupled with the ability to reach the required frequency quickly You may find at fir...

Страница 12: ...you can tune the receiver by entering frequencies directly This is very useful for quickly checking stations at known frequencies or for setting the frequency in a particular band of interest and then searching for signals with the main tuning control The keypad unit is remote from the receiver so that it can be positioned in a convenient place The keypad has 23 keys the digits 0 to 9 clock enter ...

Страница 13: ...nal from a crowded band There are four IF bandwidths offered 2 2 kHz 4 kHz 7 kHz and 10 kHz and additionally a 200 Hz wide audio filter for Morse Code reception Filters appropriate for normal conditions in each mode are selected when the receiver is first switched on 7 kHz for AM and AMS AMSL AMSU 4KHz and 2 2 kHz for USB LSB and CW The FILTER SELECT button allows this selection to be overridden T...

Страница 14: ...contents A message STO appears briefly on the display to confirm the operation Pressing MEMORY SELECT will return the receiver to normal tuning mode with the received frequency on the display The channel memory function is similar to the preview function but instead of just displaying the frequency stored in a particular memory the receiver is tuned to the memory immediately it is selected Pressin...

Страница 15: ...R SELECT button in FM mode Clock and Timers The HF250 has a 24 hour clock and two programmable timers To enter clock mode press and hold buttons 10 FAST and 2 MODE now we can set the time press button 14 ATTEN and keep it pressed and at the same time turn the tuning knob clockwise for up anticlockwise for down this will set the hours to set the minutes press and hold button 15 FILTER and by turnin...

Страница 16: ...and strings sent to the unit are in uppercase All reply markers returned are in lowercase All commands have a constant character count Frequencies are padded with leading zeros or spaces All return strings are padded with leading spaces as required A character cancels a partially entered command A CR character completes an entry 3 Commands MU n Upload set mode where n 1 to 8 MD n Download mode ret...

Страница 17: ...and obviously you should avoid spilling your coffee over the HF 250 and it won t last long if you leave it out in the rain In other words keep the receiver dry It is intended to work at normal domestic room temperatures and hot or cold extremes of temperature may affect its proper function Please make sure that the various sockets on the HF 250 are used for the intended purpose It is no use pluggi...

Страница 18: ...g noise as the tuning control is rotated It is caused by the data transfers inside the receiver being picked up by the input circuitry and it is normally not audible above the noise and signals coming from the aerial connected to the receiver As in any receiver there are a few spurious signals generated internally mostly at or slightly above the background noise level of the receiver Again these s...

Страница 19: ... IF at 45 MHz limits the signals fed to the second mixer and removes image responses from the second IF Most gain in the receiver is in the 455 kHz second IF stage where amplifiers and filters are interspersed in a chain The receiver uses ceramic multi element filters in this IF and switches in as many as possible for a selected bandwidth 2 2 kHz bandwidth 2 2 4 and 10 kHz filters 4 kHz bandwidth ...

Страница 20: ... static condition once again This method of operation virtually eliminates spurious signals from the control system being picked up by the receiver s input stages Receiver specification Frequency coverage 30 kHz to 30 MHz continuous coverage Reception modes AM LSB USB CW Narrow band FM Synchronous AMS DSB USB LSB with the DU250 option Receiver system Microprocessor controlled PLL tuning dual conve...

Страница 21: ...ry CHANNEL button Filter select Memory RECALL button MHz Down Memory STORE button MHz Up Memory STORE button Fast tune button Mode button the Tuning Memory select spin wheel Aerial select switch on rear panel FM squelch level on rear panel Aerial inputs 50n input via SO 239 socket 600 Q Input and Earth connection on spring terminals also we have a mute contact Active low High impedance active aeri...

Страница 22: ...B at 10 kHz from wanted signal 105 dB at 100 kHz from wanted signal Intermodulation effects 2 2 kHz filter At 10 kHz signal separation 3rd order intercept point 4 dBm Intermodulation free dynamic range 90 dB At 50 kHz signal separation 3rd order intercept point 13 dBm Intermodulation free dynamic range 96 dB Spurious responses Images Fixed responses Frequency stability Typical performance only not...

Страница 23: ...0 kHz filter 40 Hz to 5 2 kHz 6dB Tone control action 7 kHz filter AM mode High pass clockwise 330 Hz to 4 4 kHz 6dB Flat central 40 Hz to 4 3 kHz 6dB Low pass c clockwise 40 Hz to 1 4 kHz 6dB Distortion AM mode 1 kHz signal modulated at 70 depth With Standard AM detector THD 1 With synchronous detector THD 0 6 SSB mode 1 kHz resolved signal THD 0 2 Two signal IM products 35 dB below wanted signal...

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